This invention relates to composite brake discs generally and to carbon composite discs for use in aircraft brake assemblies in particular.
The form and construction of aircraft brake assemblies is well known and ordinarily comprises a disc stack having a plurality of annular non-rotatable stator discs interleaved with an associated number of annular rotatable rotor discs, and thrust-applying means arranged to displace the disc stack so as to bring the stator and rotor discs into frictional engagement. Typically the thrust-applying means comprises a torque tube which passes through said annular discs having a reaction flange extending radially from one end against which the discs are urged, and a ring of hydraulic `brake` cylinders displaced radially from, and rigidly attached to, the other end of said tube.
Discs of the stack must be able to generate friction forces and withstand torque loads, heat generated and the axial `thrust` load applied to them, directly or indirectly, by the brake cylinders and reaction flange. Carbon composite discs are ordinarily used because they can withstand operating temperatures in excess of 1000 degrees Celsius while maintaining the required value of friction coefficient.
Such composite materials are well known and are ordinarily of the carbon-carbon type in which carbon is reinforced by filamentary carbon possibly in the form of carbonized cloth or tape. However such materials in the form of discs, although able to withstand the required axial load if evenly distributed across a wide area, are unable to withstand high localized loads without damage. In the extreme, indention by plastic deformation or wear of the discs may cause a disc to fracture and/or break up.
Localized axial loads are usually not to be found in the body of the stack where `friction face` abutts `friction face` but at the ends of the stack where the end stator discs each have a first inwardly facing friction face and on outwardly facing load transmitting face. The load transmitting faces of the end stator discs are subject to respective axial loads from either the reaction flange or from the brake cylinders. In order to distribute the load from the reaction flange an annular reaction plate which acts as a load spreader is used. This plate is made of a suitable metal and disposed between the flange and its adjacent end stator disc. Likewise one or more annular spreader plates are disposed between the brake cylinders and their adjacent end stator disc.
Normally because of the relative size of the abutting spreader plate or reaction plate to that of the respective end stator disc, only about 50% of each disc thrust face is subject to axial load. Thus, it is likely that some warping of the disc will occur as a consequence of either this uneven loading or perhaps from the differential cooling that such abutment will precipitate. This can result in uneven wear and possible failure of the discs.
In practice carbon composite discs can readily survive cycling to and from high operating temperatures, however this is not the case with reaction or spreader plates. Permanent thermal distortion may occur in these plates which will lead to uneven application of load to the end stator discs, possibly obviating the advantage of using such plates in the first place. To overcome this disadvantage it has been suggested in U.S. Pat. No. 4,878,563 to construct an interconnected end stator disc and spreader plate, joined by a mechanical attachment which allows for thermal expansion. However, this assembly fails to overcome a further disadvantage which is that metal plates readily conduct heat from the disc pack to the hydraulic brake cylinder and piston assemblies which could lead to their premature failure.
If a brake disc could be constructed, of predominantly carbon composite, which was in one piece and was sufficiently resilient to deformation or resistant to fracture by localized loads, then the existing reaction or spreader plates would no longer be required thereby overcoming some if not all of the aforesaid disadvantages. It is an object of the invention to provide such a disc.